Damage zone modelling due to underground drilling in unsaturated argilaceous
rock with shear strain localisation
Benoît Pardoen, Université de Liège, Département ArGEnCo
Chemin des Chevreuils 1, B-4000 Liège, Sart Tilman, Belgique – b.pardoen@ulg.ac.be FRIA, F.R.S.-FNRS scholarship holder
Séverine Levasseur, Université de Liège Frédéric Collin, Université de Liège
I agree to publish the presentation after the workshop as a file at the ALERT webpage: x
□
yes□
noKeywords: strain localisation, excavation damaged zone, gallery ventilation, unsaturated soil Abstract:
Drilling of galleries induces stress perturbations that trigger damage propagation in the surrounding medium. The excavation process creates then the so-called Excavation Damaged Zone, EDZ, in which the properties are modified (Bossart et al. 2002). The prediction of the extension and of the fracture structure within this zone remains nowadays a major issue especially in the context of underground storage. Previous experimental works, at small and large scale, have highlighted that the damage around underground galleries is localised and develops mainly as strain localisation in shear band mode. In the classical finite elements framework, the strain localisation depends on the mesh size and orientation (Collin et al. 2009, Wu and Wang 2010) thus an enhanced model is needed to correctly represent the localisation behaviour. We have chosen to use the coupled second gradient local model, which includes an enrichment of the continuum with microstructure effects (Chambon et al. 1998 and 2001). Moreover, in underground structures, air ventilation inside the galleries induces a rock-atmosphere interaction that may lead to drainage and to rock desaturation close to the gallery wall. Such desaturation may influence the damage zone structure (Matray et al. 2007) and needs to be studied.
A two-dimensional plane strain state hydro-mechanical modelling of a gallery excavation including a constant air ventilation is thus performed (Lagamine code, ULg) with an air relative humidity of RH=80%. A flow model is used to reproduce transfers in partially saturated porous media. The mechanical law is an elastoplastic model with a Drucker-Prager yield surface including friction angle hardening and cohesion softening. We consider the Callovo-Oxfordian claystone as host material and the total stress field is defined according to Bishop's postulate.
The modelling provides information about the fracture structure and evolution around the gallery. The fractures represented by strain localisation bands develop during the gallery excavation and the modelling exhibits a chevron fracture pattern around the gallery (Fig. 1). The extension of the EDZ corresponds fairly well to the in situ experimental measurements of shear fractures. The convergence is anisotropic because of the shear strain localisation bands located above the gallery that implies larger displacement in the vertical direction (Fig. 2). When ventilation is applied, the desaturation of the rock close to the gallery inhibits the shear strain localisation (Fig. 1) and reduces the gallery convergence (Fig. 2). The convergence is well reproduced in the vertical direction for the modelling without ventilation (RH=100%). Furthermore, even if important changes occur during the drilling, transient effects are still visible after it. The influence of the gallery ventilation as well as of the strain localisation bands on the displacements, pore water pressures (Fig. 3) and degree of saturation is highlighted.
Nevertheless, the rock state and its properties changes still have to be considered. Characterising the influence of the rock fracturing or damage on mechanical and hydraulic properties such as permeability remains a major issue (Levasseur et al. 2009).
Selected presentations can be submitted for a special issue of the European Journal of Civil and Environmental
Engineering.
The number of the workshop oral presentations is limited. Presentations can be also accepted as a poster and published after the workshop at the ALERT webpage.
Fig. 1. Shear strain localisation bands pattern at the end of the excavation (5 days) and after it without (left) or with (right) gallery ventilation.
Fig. 2. Evolution of the vertical (left) and horizontal (right) Fig. 3. Evolution of the degree of saturation after gallery convergence during and after gallery excavation with excavation for vertical (left) and horizontal (right) comparison to experimental results (Cruchaudet et al. 2010). directions.
References:
Bossart P, Meier P M, Moeri A, Trick T, Mayor J C. Geological and hydraulic characterisation of the excavation disturbed zone in the opalinus clay of the mont terri rock laboratory. Engineering Geology, 2002, 66: 19–38.
Chambon R, Caillerie D, El Hassan N. One-dimensional localisation studied with a second grade model. European Journal of Mechanics - A/Solids, 1998, 17 (4): 637–656.
Chambon R, Caillerie D, Matsushima T. Plastic continuum with microstructure, local second gradient theories for geomaterials : localization studies. International Journal of Solids and Structures, 2001, 38: 8503–8527.
Collin F, Levasseur S, Chambon R. Numerical post failure methods in multiphysical problems. European Journal of Environmental and Civil Engineering, 2009, 13 (7-8): 983–1004.
Cruchaudet M, Noiret A, Talandier J, Gatmiri B, Armand G. OHZ en GED: EDZ initiale et évolution. Technical Report D.RP.AMFS.11.0016, Andra, 2010.
Levasseur S, Bésuelle P, Collin F, Chambon R, Charlier R, Viggiani C. EDZ in clayey rocks: which effect on permeability? In: Proceedings of European Commission - Timodaz-Teresa THMC conference, Luxembourg, 29 Sep. - 01 Oct. 2009.
Matray J M, Savoye S, Cabrera J. Desaturation and structure relationships around drifts excavated in the well-compacted Tournemire’s argillite (Aveyron, France). Engineering Geology, 2007, 90: 1–16. Wu S, Wang X. Mesh dependence and nonlocal regularization of one-dimensional strain softening
